This was a new side of Sally. When you’re twelve, your attitude, mannerisms, and word choices very much depend on who you’re talking to. When she’s talking to Mrs. Francis, it’s resentful snark, to her mother, careful but false politeness, to her father, either charming-daughter or defensiveness. (I think the most natural we’ve seen her is when she’s chatting with Glen.) But she addresses Megan as an equal, both when they’re spending sweet time together and when their relationship sours. During this whole scene, you could sense both Sally’s inner hurt feelings and genuine sense of betrayal, and that she considered herself fully in charge of the confrontation. She probes and injures Megan with that assured, mocking falseness of a girl taunting a classmate in the lunch room, not (as we’ve often seen her) with the frustration of a kid lashing back at an adult who has power over her.
This line was especially sad - she’s turning something they bonded over into another weapon in her arsenal of insults. And maybe, now that she’s seen what a talented actress Megan is, she fears that all their interactions have been fake.